The present invention relates to a semiconductor component structure with an insulating peripheral well, liable to block a voltage applied according to its main terminals.
The following description refers more particularly to a thyristor, but it will be apparent to those skilled in the art that the invention generally applies to well-type semiconductor components comprising blocking junctions on each side of a semiconductive region having a low conductivity.
The state of the art will first be recalled in connection with FIGS. 1 to 4 which show schematic cross sections of various types of thyristors. It will be apparent to those skilled in the art that the figures are not to scale but that the sizes and thicknesses of various regions are arbitrarily drawn to facilitate the legibility of the figures, as usual in the field of semiconductor components.
A thyristor is a four-layers semiconductive component comprising a cathode layer N, a base layer P.sub.1, a substrate region N.sup.- and an anode layer P.sub.2. A cathode metallization K is formed on the cathode layer N, a gate metallization G is in contact with a part of the base layer P.sub.1, and an anode metallization A is formed on the anode layer P.sub.2. The thyristor is capable of blocking a positive or negative voltage applied between its main terminals A and K and conducts a current resulting from a positive voltage applied between anode and cathode when a current is injected in the gate.
In direct polarization (positive anode, negative cathode), and in the absence of a gate signal, junction J.sub.1 between the region N.sup.- and the layer P.sub.1 is blocking. In reverse biasing (negative anode and positive cathode) junction J.sub.2 between layer P.sub.2 and region N.sup.- is blocking.
Theoretically, the breakover voltage of the blocking junction depends upon the doping gradient between layers P.sub.1 and N.sup.- on the one hand and between layers P.sub.2 and N.sup.- on the other hand, and also upon the thickness of region N.sup.-. Practically, the breakover voltages are mainly determined by the quality of the periphery of the junctions at the level of the external surfaces of the component, which will be called hereinafter apparent parameter of the junctions.
FIG. 1 shows a mesa type thyristor wherein the upper side and lower side peripheries of the thyristor are grooved, the upper groove crossing junction J.sub.1 and the lower groove crossing junction J.sub.2. Those grooves are glassivated, that is filled with a glass designated by reference 11 for the upper groove and by reference 12 for the lower groove. So, the upper and lower surfaces of the thyristor form mesas. The angle of the grooves at the apparent perimeter of junctions J.sub.1 and J.sub.2, their polishing, and the quality of the passivation are the parameters, already thoroughly studied, which determine the breakover voltage of the junctions. Presently, this type of structure provides thyristors having the highest breakover voltages (higher than one thousand volts).
However, the mesa type technology presents some basic limits; in particular, it is not compatible with the usual processes of automatic assembling. To solve this problem, well-type structures have been developed, as shown in FIGS. 2 and 3 wherein deep P-type diffusions P.sub.3 are formed the periphery of the thyristor and are electrically continuous with layer P.sub.2. The region N.sup.- is apparent at the upper surface of the thyristor and the apparent perimeters of junctions J.sub.1, J.sub.2 are on this upper surface.
FIG. 2 shows the simplest passivation mode of junctions J.sub.1 and J.sub.2 according to which, in conformity with the conventional planar technology, the apparent perimeters of the junctions are passivated by a simple oxide layer 13. To improve the voltage supported by these junctions, a metallization, called field plate, is formed above each junction. The first field plate 15 covers junction J.sub.1 and is electrically connected with the region P.sub.1 or gate G. The second field plate 16 covers junction J.sub.2 and is electrically connected to the well P.sub.3. The length of each field plate above region N.sup.- is a parameter which determines the breakover voltage of the underlying junction. However, with such structures, the breakover voltage of the junctions is not much above hundreds of volts, at best 400 volts. To improve this result, the field plates and the interval between the field plates have been covered with an additional mineral passivation layer (CVD SiO.sub.2 and silicon nitride) or non mineral passivation (polyimide) but this complicates the process in exchange for a relatively low improvement. The voltage breakdown is then at best 600 volts.
In the structures of FIGS. 1 and 2, there is no stray current problem, that is, when the junctions are reverse biased, the stray current between the anode and the cathode is lower than one microampere.
FIG. 3 shows a known solution to improve the breakover voltage of a well-type thyristor. The apparent surface of region N.sup.- is grooved so as to cut junctions J.sub.2 and J.sub.1. The groove is filled with a glass 18. Then, a satisfying breakover voltage is obtained, in the same range as obtained with mesa type thyristors. However, a new problem arises, i.e., a relatively high stray current appears when the thyristor is blocked in the direct or reverse direction, in particular when the device is hot. This stray current is not stable and sometimes reaches high values such as tens, or hundreds of microamperes.
The stray current in the structure of FIG. 3 is likely caused by fixed or mobile negative charges in glass 18.
In the case of mesa structures as shown in FIG. 1, the effect of the charges is to increase the breakover voltage of each of the junctions J.sub.1 and J.sub.2 by locally compensating the N type concentration of the silicon intrinsic region (N.sup.-).
In the case of FIG. 3, the presence of those negative charges in glass 18 causes a reversal of the conductivity type of the surface part of region N.sup.- and creates a channel, causing the presence of a stray current. To palliate this drawback, it has been suggested to use a structure as shown in FIG. 4, comprising, at the center of the surface portion of the N.sup.- region, a N.sup.+ diffusion region 19 called a channel stop. Such a channel stop efficiently reduces the stray current which becomes lower than one microampere. However, such a structure has many drawbacks, that is, on the one hand, its difficult implementation which implies additional manufacturing steps and, on the other hand, the fact that the distance e between the limit of each junction and the N.sup.+ region 19 has to be higher than the thickness of region N.sup.-.
A structure of the type disclosed in FIG. 4 is for example disclosed in U.S. Pat. No. 4,148,053 which also teaches a double groove structure. This patent also suggests the use of field plates for their conventional function of spreading the field lines and not for a reduction of the stray current. An apparent drawback of the structure disclosed in this patent is that it occupies a large area.